Bel Canto uses a rather old-fashioned device (common in TV dramas of the 1960s) to settle accounts. The novel has a final chapter labelled "Epilogue". It has had its catastrophe - its disastrous resolution. Its drama of romance, comedy and day-dreaming is abruptly concluded. The siege of the vice-president's residence is ended when troops storm in, killing many of the characters whom we have come to know. The violence and finality is done rather well, the narrative taking on the numbed surprise of the characters. We are left in a state of shock.

But then we have an epilogue. It is six months later and two former hostages are marrying each other. The tragic conclusion behind us, we now have an ending conventional in comedy. The odd thing is that the bride and groom - Roxane, the opera singer, and Gen, the translator - each had a brief, secret affair with someone else earlier in the novel. And each of their lovers was killed, one on purpose and one by mistake, in its final scene. Patchett's epilogue seems designed to rescue something from her tale.

An epilogue risks persuading the reader that the author feels uneasy about the ending of the narrative. After the conclusion of a fable, an epilogue is a proper summary, where the moral might be gracefully pointed. At the end of a novel, however, it should be different. A novel is too much like life, we might think, to be reducible to a moral. Its epilogue is more like an afterthought, something that could not be got into the story.

Until the 20th century, it was common for plays, especially comedies, to have epilogues. (Epilogue means, in Greek, "a speech added on".) One of the actors would step forward out of the play's true ending to comment, often wittily, on its events and absurdities. The audience would be asked for its indulgence. Done well, it was, and still can be, a way of confirming a bond between performer and spectator. In a novel, an epilogue can look like the novelist distrusting the reader.

Yet the novel is also the genre that would have us believe that its characters might have a life beyond its pages. Jane Austen used to enjoy discussing with her family the after-fates of Elizabeth and Darcy, Emma, Mr Wodehouse and Mr Knightley. Samuel Richardson ended his great tragic novel, Clarissa, with short accounts of the apt destinies of all his characters. It is hard to find the earliest novel with a declared "epilogue" (this is the kind of thing that academic criticism never tells you), but this formal after-story became a feature of fiction in the mid-19th century.

Herman Melville's Moby-Dick has a brilliant little example. The whaling ship, the Pequod, has been sunk by the great white whale in an extraordinary final scene. The crew is lost. But then there is an "Epilogue". "The drama's done. Why then does any one step forth?" Melville reminds us of stage convention, as Ishmael tells us how he was providentially saved. If he had not been, of course, the story could not have been told.

Some great Victorian novels have epilogues. The "Epilogue" of George Eliot's Adam Bede takes us forward by seven years to show the appropriate fates of its chief characters. Adam, the artisan hero, has prospered. Arthur Donnithorne, the rakish young squire, has been punished with illness and exile and returns, thin and reformed. Dinah, the good woman, has been rewarded with beautiful children. Eliot's moral scheme is secure.

Wilkie Collins's The Moonstone has an "Epilogue" that satisfyingly brings his narrative full circle. The mystery of the Moonstone's theft has been solved. In the epilogue, we hear how this fabulous diamond is restored to the Indian idol from which it was first taken. Later detective fiction also uses epilogue. Most of Colin Dexter's Inspector Morse novels, for instance, have epilogues. In the calm after the dénouement, accounts are settled and Morse himself is pictured rueful and solitary, a melancholy figure until another murder can animate him again.

Why, then, does Patchett need an epilogue? She wants to show that her characters have truly been transformed by their strange experience, that they have been redeemed by the events that she has plotted. Perhaps her epilogue shows that she all too clearly wants us to believe this too.

· John Mullan is senior lecturer in English at University College London. Have your say about Bel Canto on the Guardian talkboards at booktalk.theguardian.com or write to The Review, The Guardian, 119 Farringdon Road, London EC1R 3ER.